An electric power steering apparatus which provides a steering mechanism of a vehicle with a steering assist torque (an assist torque) by means of a rotational torque of a motor, applies a driving force of the motor as the assist torque to a steering shaft or a rack shaft by means of a transmission mechanism such as gears or a belt through a reduction mechanism. In order to accurately generate the assist torque, such a conventional electric power steering apparatus (EPS) performs a feedback control of a motor current. The feedback control adjusts a voltage supplied to the motor so that a difference between a steering assist command value (a current command value) and a detected motor current value becomes small, and the adjustment of the voltage applied to the motor is generally performed by an adjustment of a duty ratio of a pulse width modulation (PWM) control.
A general configuration of a conventional electric power steering apparatus will be described with reference to FIG. 1. As shown in FIG. 1, a column shaft (a steering shaft) 2 connected to a steering wheel (a handle) 1, is connected to steered wheels 8L and 8R through reduction gears 3, universal joints 4a and 4b, a rack and pinion mechanism 5, and tie rods 6a and 6b, further via hub units 7a and 7b. Further, the column shaft 2 is provided with a torque sensor 10 for detecting a steering torque Th of the steering wheel 1, and a motor 20 for assisting the steering force of the steering wheel 1 is connected to the column shaft 2 through the reduction gears 3. Electric power is supplied to a control unit (ECU) 30 for controlling the electric power steering apparatus from a battery 13, and an ignition key signal is inputted into the control unit 30 through an ignition key 11. The control unit 30 calculates a current command value of an assist (steering assist) command based on a steering torque Th detected by the torque sensor 10 and a vehicle speed Vel detected by a vehicle speed sensor 12, and controls a current supplied to the motor 20 based on a voltage control command value Vref obtained by performing compensation and so on with respect to the current command value. Moreover, a steering angle sensor 14 is not essential and it also may be not to provide.
A controller area network (CAN) 50 to send/receive various information and signals on the vehicle is connected to the control unit 30, and it is also possible to receive the vehicle speed Vel from the CAN. Further, a Non-CAN 51 is also possible to connect to the control unit 30, and the Non-CAN 51 sends and receives a communication, analogue/digital signals, electric wave or the like except for the CAN 50.
The control unit 30 mainly comprises a CPU (or an MPU or an MCU), and general functions performed by programs within the CPU are shown in FIG. 2.
Functions and operations of the control unit 30 will be described with reference to FIG. 2. As shown in FIG. 2, the steering torque Th detected by the torque sensor 10 and the vehicle speed Vel from the vehicle speed sensor 12 (or, from the CAN 50) are inputted into a current command value calculating section 31 to calculate a current command value Iref1. The current command value calculating section 31 calculates the current command value Iref1, which is a control target value of a current to supply to the motor 20, by means of an assist map or the like based on the steering torque Th and the vehicle speed Vel. The calculated current command value Iref1 is inputted into a current limiting section 33 through a subtracting section 32A and a current command value Irefm of which a maximum current is limited, is inputted into a subtracting section 32B. In the subtracting section 32B, a deviation (error) I (=Irefm−Im) between the limited current command value Irefm and a motor current value Im being fed-back is calculated, and the deviation I is inputted into a PI-control section 35 to improve a characteristic of the steering operation. The voltage control command value Vref of which the characteristic is improved is inputted into the PWM-control section 36, and further the motor 20 is PWM-driven through the inverter 37 as a driving section. The motor current Im is detected by a motor current detector 38 and the detected motor current Im is fed-back to the subtracting section 32B. The inverter 37 uses FETs as driving elements and comprises a bridge circuit of the FETs.
Further, a compensation signal CM from a compensating section 34 is added to the adding section 32A, which performs compensation of a system by adding the compensation signal CM and improves convergence, an inertial characteristic and so on. The compensating section 34 adds a self-aligning torque (SAT) 343 and the inertia 342 by an adding section 344, moreover, adds the convergence 341 to an addition result by an adding section 345, and makes the compensation signal CM of the addition result of the adding section 345.
In the case that the motor 20 is a three-phase (U-phase, V-phase and W-phase) brushless DC-motor, details of the PWM-control section 36 and the inverter 37 is a configuration such as shown in FIG. 3. That is, the PWM-control section 36 comprises a duty calculating section 36A that calculates PWM-duty command values D1 to D6 of three phases (U-phase, V-phase and W-phase) in synchronous with a PWM-carrier CS by using a predetermined expression based on the voltage control command value Vref, and a gate driving section 36B that drives respective gates of the FET1 to FET6 by the PWM-duty command values D1 to D6, compensates dead times and turns ON/OFF. The inverter 37 comprises a three-phase bridge (High-side FET1 to FET3 and Low-side FET4 to FET6) and drives the motor 20 by being ON/OFF-operated with the PWM-duty command values D1 to D6.
In such the electric power steering apparatus, it is necessary to detect the respective phase currents of the motor 20 and to feedback, and a current detecting circuit of three-shunt type as shown in FIG. 4 is well known. That is, shunt resistances RS1 to RS3 are respectively inserted at down-stream three-phases in the inverter 37, fall voltages due to the shunt resistances RS1 to RS3 are measured, the measured voltages are respectively converted into digital values at A/D converters 37U to 37W and simultaneously converted into current values, and detected three-phase motor currents Iu to Iw are fed back.
In the current detecting system of the down-stream three-shunts, it is necessary to detect at least two-phase currents with a simultaneous sampling at a timing when the Low-side FETs are “ON”. If the two-phase currents could be detected, it is possible to detect the rest one-phase current due to a relation “Iu+Iv+Iw=0”. Further, in a case that the duty is saturated (maximum) (ON-time of the Low-side FET is minimum), there is a possibility that its phase current detection becomes impossible due to that a current does not flow in the shunt resistances of the Low-side. Thus, under the consideration of the detection property at a time of the duty saturation, it is necessary to detect at least the two-phase currents of which the duties are not saturated, by judging a phase that the duty changing during the motor rotation is saturated.
For satisfying the above conditions, the following methods (1) to (4) are generally performed:    (1) A method to provide three A/D converters within an MCU as shown in FIG. 4, to input 3-phase detected-currents into respective individual units, and to simultaneously sample 2-phase currents detected-signals of which the duties are not saturated.    (2) A method to provide two A/D converters within an MCU, to provide a multiplexer outside of the MCU, to output 2-phase current detected-signals of which the duties are not saturated among 3-phases, to individual units within the MCU, and to simultaneously sample the 2-phase current detected-signals.    (3) A method to adopt an A/D converter with a sample-hold circuit and to simultaneously sample-hold 3-phase current detected-signals being connected to the A/D converter and to detect.    (4) A method to provide a sample-hold circuit outside of an MCU and to A/D-converts 3-phase current detected-signals which are simultaneously sample-held at a predetermined timing.